1. Field of Invention
The present invention relates to neutron bombardment nuclear well logging.
2. Description of the Prior Art
At one time, it had generally been considered incompatible to perform a simultaneous log of both the silicon and oxygen components of earth formations in the vicinity of a well borehole.
Detection methods have centered on measuring either oxygen or silicon independently, since for reasonable source to detector spacings, oxygen activation required a fast logging speed due to the short (7.35 seconds) half life of gamma radiation from the radioactive isotope nitrogen-16. Conversely, silicon activation required a slow logging speed because of the longer (2.3 minutes) half life of gamma radiation from the radioactive isotope aluminum-28. Problems thus arose in attempting to log silicon and oxygen simultaneously by other techniques, because of the conflicting spacing and logging speed requirements due to the large difference in half lives of their radioactive isotopes of interest.
U.S. Pat. No. 3,943,362, of common ownership herewith, related to simultaneous determination of the relative silicon and oxygen content of an earth formation surrounding a well borehole. A neutron source was activated to bombard the formation, causing among others the O.sup.16 (n, p)N.sup.16 reaction where oxygen was present and the Si.sup.28 (n, p)Al.sup.28 reaction where silicon was present. A single detector and photomultiplier tube with associated electronics formed and furnished an electrical signal, representative of detected gamma rays from these nuclear reactions, which was transmitted to a pulse height analyzer or analyzers in a surface circuit. The pulse height analyzer counted pulses in two energy windows, one for an oxygen peak at 6.13 MeV and one for an aluminum peak at 1.78 MeV. However, the energy windows set for aluminum also encompassed Compton scattering low energy gamma ray events, particularly from the 6.13 MeV gamma rays. Accordingly, an initial determination of background count rate was required, using a specified calibration procedure to compensate for the Compton scattering events.
U.S. Pat. No. 3,928,762 of common ownership herewith detected and measured, among other measurements, the hydrogen/oxygen ratio of earth formations using nuclear well logging methods. U.S. Pat. No. 3,959,648 related to an apparatus for detecting and measuring the same or two different conditions with separate detectors in nuclear well logging. Other prior art of which applicants are aware having two or more detectors are U.S. Pat. Nos. 2,508,772 and 2,963,587. In the first of these patents, radiation was logged at plural detectors spaced differing distances from a source so that decay of radiation intensity as a function of distance, an indicator of the nature of the formation, was obtained. In the other of these patents, the plural detectors were moved to the formation of interest different times after neutron bombardment, so that differences of count rate as a function of time gave an indication of decay rate, from which radiation half life is indicated. From the indicated half life, it was proposed to identify elements in the formation.